There’s a lot of jitter in the New Orleans numbers, but a couple of things seem clear:

Through most of the period 1851–75, there’s much more literary attention paid to New Orleans than to Chicago.

Interest in Chicago picks up meaningfully after about 1870.

Interest in New Orleans wanes a bit around the same time, but only to the extent that the two cities occur at about equal rates in the last few years of the corpus.

…

[And in sum:] I’m sure there’s some novelty-driven interest in emerging cities and demographic changes, but at least in the case of Chicago and New Orleans, it doesn’t appear to be the dominant factor driving literary attention.

This is also a chance to put in a plug for Scalable Reading, both the blog and the concept. Well worth a read, I think, my own contribution notwithstanding.